1. Field of the Invention
The present invention relates to a method for reducing or eliminating parasitic oscillations [i.e. amplified spontaneous emissions (ASE)] in solid state laser materials. More specifically, it relates to an article comprised of a solid state laser gain material center which has bonded to its peripheral edge using a bonding agent a laser light absorbing material. All three components each have specific chemical and physical properties.
2. Description of Related Art
The architecture of most short-pulse, high-peak power neodymium (Nd) glass laser systems consists of a master oscillator followed by a series of power amplifiers. An example of one such system is the 100 kiloJoule (kJ) (pulsed) Nova laser at the Lawrence Livermore National Laboratory (LLNL), Livermore, Calif. In these laser systems the large power amplifiers contain disks of Nd-doped glass that are optically pumped by a series of xenon (Xe) flashlamps. If not properly designed, the stored energy density in laser glass disks decreases as the product of the disk size (D) and small-signal-gain coefficient (.alpha.) (i.e. .alpha..multidot.D) increases. This performance reduction is caused by the complementary effects of amplified spontaneous emission (ASE) and parasitic oscillations that can occur in the Nd:glass disks. (See, for example, "Fluorescence Amplification and Parasitic Oscillation Limitations in Disk Lasers", by J. B. Trenholme, NRL Memorandum Rep. 2480, July, 1972; J. E. Swain, et al., J. Appl. Phys., 40, p. 3973 (1969); and J. M. McMahon et al., IEEE J. Quantum Electron. QE-9, p. 992 (1973)).
Edge claddings are used on laser disks to absorb the amplified spontaneous emission (ASE) and to suppress the onset of parasitic oscillations that would otherwise reduce the stored energy. In general, these claddings consist of a material that is refractive index matched to the laser glass and which contains a dopant that absorbs at the laser (ASE) frequency. A number of different materials have been used for cladding, ranging from sprayed-on glass frits to liquids to castings of monolithic glass. (See, for example, G. Dube and N. L. Boling, in Applied Optics, Vol. 13, p. 699 (1974); S. Guch, Jr., in Applied Optics, Vol. 15, p. 1453 (1976); and D. Milam, C. W. Hatcher and J. H. Campbell, in "Platinum Particles in the Nd:doped Disks of Phosphate Glass in the Nova Laser", in Laser Induced Damage in Optical Materials: 1985: Proceedings of the Boulder Damage Symposium, Nov. 1985, Boulder, Colo.) In the 100-kJ Nova pulsed laser system, claddings of monolithic glass doped with ionic copper that absorbs at 1 micrometer have been used. Although the performance of this latter cladding is excellent, it is very expensive to apply. Further, it is known to induce some degree of residual stress near the disk edges that can potentially affect beam optical quality.
When the NOVA laser at LLNL was commissioned in 1985, it was found the phosphate laser glass contained a large number of microscopic Pt inclusions. These inclusions produced fractures within the glass of the power amplifiers when subjected to the high fluence Nova beam. Consequently, the glass was replaced with a newly developed Pt-inclusion-free glass.
In research reported by J. E. Murray et al., in "Silicone Rubber Edge Claddings for Laser Disk Amplifiers", in CLEO 84, Paper No. THF-2 (June, 1984), disk amplifiers were produced having edge claddings to prevent feedback of ASE. In particular, a room temperature-vulcanized (RTV) silicone rubber was poured about the peripheral edge of the laser disk and plates of filter glass were embedded in the rubber to absorb ASE. This silicone rubber-based edge cladding was successfully employed in a developmental amplifier, and it survived several thousand shots without degradation. As such it met most of the requirements of a low-cost, functional edge cladding which could be used on a large laser system. It was potentially cheap, because the materials were inexpensive, and it could be applied at room temperature. It performed well as an edge cladding, because the cured silicone rubber was water-clear, and its refractive index could be adjusted over the range 1.42 to 1.54, which included most laser glasses. Its major difficulty was that the cured rubber surface could not be cleaned adequately for the high energy laser environment. Dirt particles clung to the rubber surface, and the rubber itself was somewhat soft and crumbly so that it could not be wiped clean. Since components of a high energy laser system must be extremely clean to prevent damage from the laser beam, the silicone rubber itself prevented this cladding type from being used in high energy laser systems.
It is desirable to have a general low-cost method to clad the peripheral edge of glass laser disks with a laser light absorbing material capable of withstanding higher fluences and exhibiting higher moduli to and in grinding and finishing operations. The present invention provides such a method and article.